The present invention relates to an improved photohardenable composition of the type described in U.S. Pat. No. 4,772,541 containing a photohardenable compound, a complex of a cationic dye and a borate as a photoinitiator, and a borate salt.
U.S. Pat. No. 4,772,541 is incorporated herein by reference. This patent discloses photohardenable compositions which contain a dye complex of the formula (I): ##STR1## where D+ is a cationic dye; and R.sup.1, R.sup.2, R.sup.3, and R.sup.4 are independently selected from the group consisting of alkyl, aryl, alkaryl, allyl, aralkyl, alkenyl, alkynyl, alicyclic and saturated or unsaturated heterocyclic groups. As noted in the patent, these groups may be substituted or unsubstituted.
Useful dyes form photoreducible but dark stable complexes with borate anions and can be cationic methine, polymethine, triarylmethane, indoline, thiazine, xanthene, oxazine and acridine dyes. More specifically, the dyes may be cationic cyanine, carbocyanine, hemicyanine, rhodamine and azomethine dyes.
Preferred dyes are cyanine dyes of the formula (II) ##STR2## where n is 0, 1, 2 or 3; R is an alkyl group and preferably an alkyl group having 4 to 10 carbon atoms; and y is --CH.dbd.CH--, &gt;NCH.sub.3,&gt;C(CH.sub.3).sub.2, O, S, or Se
The borate anion is designed such that the borate radical generated upon exposure to light and after electron transfer to the dye (Eq. 1) readily dissociates with the formation of a radical as follows: EQU BR.sub.4 .fwdarw.BR.sub.3 +R. (Eq. 1)
For example particularly preferred anions are triphenylbutylborate, trianisylbutylborate, and triphenylsecbutyl borate anions because they readily dissociate to triphenylborane or trianisylborane and a butyl radical. As explained in U.S. Pat. No. 4,772,541, the Weller equation is useful in identifying useful complexes.
To enhance the film speed of photohardenable compositions described in the aforementioned patent, compounds such as N,N-dialkylanilines and more particularly diisopropyldimethylaniline may be added to the composition. Thiols such as mercaptobenzothiazole may also be added to the composition.